As you can see in Apple Documentation for MPMediaItem http://developer.apple.com/library/ios/#documentation/mediaplayer/reference/MPMediaItem_ClassReference/Reference/Reference.html#//apple_ref/occ/cl/MPMediaItem, you access all the properties through enumerateValuesForProperties:usingBlock:
or valueForProperty:
, but there isn't a property for Date Added or Purchased Date informations.
This doesn't mean that the data isn't stored somewhere (surely it is): this just means that you cannot access that information through public APIs, maybe for privacy issues.
I don't know anyway if there is a way to obtain this information through some private API calls, but if it exists you can't use it for AppStore apps because it will cause you a rejection during review process.